Oxadiazolyl-benzene azo hydroxy-pyridone dyes for thermal transfer printing a yellow print

ABSTRACT

Azo dyes useful for thermotransfer printing have the formula ##STR1## where R 1  and R 2  are each H; alkyl, alkoxyalkyl, alkanoyloxyalkyl, alkoxycarbonyloxyalkyl, alkoxycarbonylalkyl, haloalkyl, hydroxyalkyl or cyanoalkyl each of up to 20 carbon atoms and each of which may be substituted by phenyl, C 1  -C 4  -alkylphenyl, C 1  -C 4  -alkoxyphenyl, halophenyl, benzyloxy, C 1  -C 4  -alkylbenzyloxy, C 1  -C 4  -alkoxybenzyloxy, halobenzyloxy, halogen, hydroxyl or cyano; phenyl or cyclohexyl which may each be substituted by C 1  -C 15  -alkyl, C 1  -C 15  -alkoxy, halogen or benzyloxy; or a radical II 
     
         [--W--O].sub.n --R.sup.4                                   II 
    
     where W is identical or different C 2  -C 5  -alkylene, n is 1-6, R 4  is C 1  -C 4  -alkyl or phenyl which may be substituted by C 1  -C 4  -alkyl or C 1  -C 4  -alkoxy, R 3  is H, NH 2 , OH or C 1  -C 3  -alkyl, X is H, Cl, Br, No 2 , methyl, phenoxy, tolyloxy, dimethylphenyloxy, chlorophenoxy or C 1  -C 4  -alkoxy, Y is H, Cl or Br, and Z is H, acetyl, carbamoyl or cyano.

The present invention relates to a process for thermotransfer printingwherein the transfer dye is an azo dye of the general formula I ##STR2##in which the substituents have the following meanings: R¹ and R² areeach

hydrogen;

alkyl, alkoxyalkyl, alkanoyloxyalkyl, alkoxycarbonyloxyalkyl,alkoxycarbonylalkyl, haloalkyl, hydroxyalkyl or cyanoalkyl, which mayeach contain up to 20 carbon atoms and be substituted by phenyl, C₁ -C₄-alkylphenyl, C₁ -C₄ -alkoxyphenyl, halophenyl, benzyloxy, C₁ -C₄-alkylbenzyloxy, C₁ -C₄ -alkoxybenzyloxy, halobenzyloxy, halogen,hydroxyl or cyano;

phenyl or cyclohexyl, which may each be substituted by C₁ -C₁₅ -alkyl,C₁ -C₁₅ -alkoxy, halogen or benzyloxy; or

a radical of the general formula II

    [--W--O].sub.n --R.sup.4                                   II

where

W is identical or different C₂ -C₆ -alkylene,

n is from 1 to 6 and

R⁴ is C₁ -C₄ -alkyl or phenyl which may each be substituted by C₁ -C₄-alkyl or C₁ -C₄ -alkoxy;

R³ is hydrogen, amino, hydroxyl or alkyl;

X is hydrogen, chlorine, bromine, nitro, methyl, phenoxy, tolyloxy,dimethylphenyloxy, chlorophenoxy or C₁ -C₄ -alkoxy;

Y is hydrogen, chlorine or bromine; and

Z is hydrogen, acetyl, carbamoyl or cyano;

and also specifically to a process for transferring this azo dye bydiffusion from a transfer to a plastic-coated sheet of paper with theaid of a thermal printing head.

The technique of thermotransfer printing is common knowledge; suitableheat sources besides lasers and IR lamps are in particular thermalprinting heads capable of emitting short heat pulses lasting fractionsof a second.

In this preferred embodiment of thermotransfer printing, a transfersheet which contains the transfer dye together with one or more binders,a substrate material and possibly further assistants such as releaseagents or crystallization inhibitors is heated from the back with thethermal printing head, causing the dye to migrate out of the transfersheet and to diffuse into the surface coating of a receiving medium, forexample into the plastic coat of a coated sheet of paper.

The essential advantage of this process is that the amount of dye to betransferred (and hence the color gradation) can be controlled in aspecific manner by adjusting the energy supplied to the thermal printinghead.

Thermotransfer printing is in general carried out using the threesubtractive primaries yellow, magenta and cyan (with or without black),and the dyes used must have the following properties to ensure optimalcolor recording: ready thermal transferability, little tendency tomigrate within or out of the surface coating of the receiving medium atroom temperature, high thermal and photochemical stability and alsoresistance to moisture and chemicals, no tendency to crystallize onstorage of the transfer sheet, a suitable hue for subtractive colormixing, a high molar absorption coefficient, and ready industrialavailability.

It is very difficult to meet all these requirements at one and the sametime. In particular, the yellow dyes used to date have beenunconvincing. This is also true of the azopyridones described andrecommended for thermotransfer printing in EP-A-247 737, JP-A-12393/1986, JP-A-152 563/1985, JP-A-152 563/1985, JP-A-262 191/1986 andJP-A-244 595/1986, which are similar to the compounds I but do not carryan oxathiazole radical on the phenyl ring, which may be unsubstituted orsubstituted.

The azo dyes I are themselves known per se or obtainable by knownmethods (EP-B-111 236).

It is an object of the present invention to find suitable yellow dyesfor thermotransfer printing which come closer to the required propertyprofile than the prior art dyes.

We have found that this object is achieved by the azo dyes of theformula I defined at the beginning.

We have also found a process for transferring azo dyes from a transferto a plastic-coated medium by diffusion with the aid of a thermalprinting head, which comprises using for this purpose a transfer onwhich there is or are situated one or more of the azo dyes I defined atthe beginning.

We have further found preferred embodiments of this process, whichcomprise using dyes of the formula Ia ##STR3## having the above-definedmeanings of substituents R¹, R² and X and dyes of the formula Ib##STR4## where X is as defined at the beginning and R^(1') and R^(2')are each:

hydrogen;

alkyl, alkoxyalkyl, alkanoyloxy or alkoxycarbonylalkyl, which may eachcontain up to 15 carbon atoms and be substituted by phenyl, C₁ -C₄-alkylphenyl, C₁ -C₄ -alkoxyphenyl, hydroxyl or cyano; or

a radical of the general formula III

    [--(CH.sub.2).sub.3 --O].sub.p --(CH.sub.2).sub.2 --O].sub.q --R.sup.4'III

where p is 0 or 1, q is from 1 from 4, and R^(4') is C₁ -C₄ -alkyl orphenyl.

R¹ and R² are each preferably alkyl: methyl, ethyl, propyl, isopropyl,butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, sec-pentyl,tert-pentyl, hexyl, 2-methylpentyl, heptyl, octyl, 2-ethylhexyl, theisooctyl isomer mixture, nonyl, decyl, the isononyl and isodecyl isomermixtures, and also undecyl or dodecyl.

In addition, it is also possible to use for example tridecyl, theisotridecyl isomer mixture, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, nonadecyl or icosyl.

Any alkyl may also be substituted by phenyl; specific examples, wherePh=phenyl, are:

--CH₂ --Ph, --CH(CH₃)--Ph, --(CH₂)₂ --Ph,

--(CH₂)₄ --CH(CH₃)--Ph--3--CH₃,

--(CH₂)₃ --CH(C₄ H₉)--Ph--3-C₄ H₉,

--(CH₂)₆ --Ph--4--O--CH₃,

--CH(C₂ H₅)--(CH₂)₃ --Ph--3--O--C₂ H₅ and

--CH(C₂ H₅)--(CH₂)₃ --Ph--3--Cl.

When R¹ or R² is alkoxyalkyl of the preferred formula II, suitable W isfor example 1,2- or 1,3-propylene, 1,2-, 1,3-, 1,4- or 2,3-butylene,pentamethylene, hexamethylene or 2-methylpentamethylene, in particularethylene, R⁴ is in particular methyl, ethyl, propyl, butyl or phenylwhich may be substituted by methyl(oxy), ethyl(oxy), propyl(oxy) orbutyl(oxy). Particularly preferred groups II are for example:

--(CH₂)₂ --O--CH₃, --(CH₂)₂ --O--C₂ H₅, --(CH₂)₂ --O--C₃ H₇, --(CH₂)₂--O--C₄ H₉, --(CH₂)₂ --O--Ph, --(CH₂)₂ --O--CH₂ --Ph,

--[(CH₂)₂ --O]₂ --CH₃, --[(CH₂)₂ --O]₂ --Ph, --[(CH₂)₂ --O]₂-Ph-4--O--C₄ H₉,

--[(CH₂)₂ --O]₃ --C₄ H₉, --[(CH₂)₂ --O]₃ --Ph, --[(CH₂).sub.₂ --O]₃--Ph--3--C₄ H₉,

--[(CH₂)₂ --O]₄ --CH₃ and

--(CH₂)₃ --O--(CH₂)₂ --O--Ph.

Further preferred groups II are for example:

--(CH₂)₃ --O--CH₃, --(CH₂)₃ --O--C₂ H₅, --(CH₂)₃ --O--C₃ H₇, --(CH₂)₃--O--C₄ H₉,

--(CH₂)₃ --O--Ph,

--CH₂ --CH(CH₃)--O--CH₃, --CH₂ --CH(CH₃)--O--C₂ H₅, --CH₂--CH(CH₃)--O--C₃ H₇, --CH₂ --CH(CH₃)--O--C₄ H₉, --CH₂ --CH(CH₃)--O--Ph,--(CH₂)₄ --O--CH₃, --(CH₂ ₄ --O--C₂ H₅, --(CH₂)₄ --O--C₄₅ H₉, --(CH₂)₄--O--Ph,

--(CH₂)₄ --O--CH₂ --Ph--2--O--C₂ H₅, --(CH₂)₄ --O--C₆ H₁₀ --2--C₂ H₅,

--[(CH₂)₄ --O]₂ --C₂ H₅, --[(CH₂)₂ --CH(CH₃)--O]₂ --C₂ H₅,

--(CH₂)₅ --O--CH₃, --(CH₂)₅ --O--C₂ H₅, --(CH₂)₅ --O--C₃ H₇, --(CH₂)₅--O--Ph,

--(CH₂)₂ --CH(C₂ H₅)--O--CH₂ --Ph--3--O--C₄ H₉, --(CH₂)₂ --CH(C₂H₅)--O--CH₂ --Ph--3-Cl,

--(CH₂)₆ --O--C₄ H₉, --(CH₂)₆ --O--Ph--4--O--C₄ H₉ and

--(CH₂)₃ --CH(CH₃)--CH₂ --O--C₄ H₉.

In addition, it is also possible to use the following alkoxyalkylgroups:

--(CH₂)₈ --O--CH₃, --(CH₂)₈ ----O--C₄ H₉, --(CH₂)₈ --O--CH₂ --Ph--3--C₂H₅,

--(CH₂)₄ --CH(Cl)--(CH₂)₃ --O--CH₂ --Ph--3--CH₃ and

--(CH₂)₃ --CH(C₄ H₉)--O--CH₂ --Ph--3--CH₃.

Suitable alkanoyloxyalkyl, alkoxycarbonyloxyalkyl or alkoxycarbonylalkylR¹ or R² is for example:

--(CH₂)₂ --O--CO--CH₃,

--(CH₂)₃ --O--CO--(CH₂)₇ --CH₃,

--(CH₂)₂ --O--CO--(CH₂)₃ --Ph--2--O--CH₃,

--CH(CH₂ --Ph--3--CH₃)--O--CO--C₄ H₉ or --(CH₂)₄ --O--CO--(CH₂)₄ --CH(C₂H₅)--OH;

--(CH₂)₂ --O--CO--O--CH₃,

--(CH₂)₃ --O--CO--O--(CH₂)₇ --CH₃,

--CH(C₂ H₅)--CH₂ --O--CO--O--C₄ H₉,

--(CH₂)₄ --O--CO--O--(CH₂)₂ --CH(CH₃)--O--Ph--3--CH₃ or --(CH₂)₅--O--CO--O--(CH₂)₅ --CN;

--(CH₂)₂ --CO--O--CH₃,

--(CH₂)₃ --CO--O--C₄ H₉,

--(CH₂)₃ --CH(CH₃)--CH₂ --CO--O--C₄ H₉,

--(CH₂)₃ --CH(C₄ H₉)--CH₂ --CO--O--C₂ H₅,

--(CH₂)₂ --CO--O--(CH₂)₅ --Ph,

--(CH₂)₄ --CO--O--(CH₂)₄ --Ph--4-C₄ H₉,

--(CH₂)₃ --CO--O--(CH₂)₄ --O--Ph--3--O--CH₃,

--(CH₂)₂ --CH(CH₂ OH)--(CH₂)₂ --CO--O--C₂ H₅,

--CH(C₂ H₅)--CH₂ --CO--O--(CH₂)₄ --OH or

--(CH₂)₃ --CO--O--(CH₂)₆ --CN.

It is also possible to use for example the following haloalkyl,hydroxyalkyl or cyanoalkyl groups as R¹ or R² :

--(CH₂)₅ --Cl, --CH(C₄ H₉)--(CH₂)₃ --Cl oder --(CH₂)₄ --CF₃ ;

--(CH₂)₂ --CH(CH₃)--OH, --(CH₂)₂ --CH(C₄ H₉)--OH oder --CH(C₂H₅)--(CH₂)₉ --OH;

--(CH₂)₂ --CN, --(CH₂)₃ --CN, --CH₂ --CH(CH₃)--CH(C₂ H₅)--CN, --(CH₂)₆--CH(C₂ H₅)--CN and

--(CH₂)₃ --CH(CH₃)--(CH₂)₂ --CH(CH₃)--CN.

or

Phenyl or cyclohexyl which may be present as R¹ or R² are for example:

--Ph, --Ph--3--CH₃, --Ph--4--(CH₂)₁₀ --CH₃, --Ph--3--(CH₂)₅--CH(CH₃)--CH₃,

--PH--4--O--C₄ H₉, --Ph--4--(CH₂)₅ --CH(C₂ H₅)--CH₃, --Ph--4--O--CH₂--Ph and --Ph--4--Cl;

--C₆ H₁₀ --4--CH₃, --C₆ H₁₀ --4--C₁₀ H₂₁, --C₆ H₁₀ --3--O--C₄ H₉,

--C₆ H₁₀ --3--O--(CH₂)₄ --CH(C₂ H₅)--CH₃ oder --C₆ H₁₀ --4--Cl.

A particularly preferred R³ is methyl, but it is also possible to useethyl, propyl or isopropyl as well as hydrogen, amino or hydroxyl.

X and Y are each preferably hydrogen or nitro or else chlorine orbromine. Further suitable X is methyl, methyloxy, ethyloxy, propyloxy,butyloxy, phenoxy, tolyloxy or dimethylphenoxy.

Particularly preferred Z is cyano, but it is also possible to usehydrogen, acetyl or carbamoyl.

The dyes I to be used according to the present invention are notable forthe following properties compared with the prior art yellowthermotransfer printing dyes: readier thermal transferability despitethe relatively high molecular weight, improved migration properties inthe receiving medium at room temperature, significantly higherlightfastness, better resistance to moisture and chemicals, bettersolubility in printing ink preparation, higher color strength, andreadier industrial accessibility.

In addition, the azo dyes I exhibit a distinctly better purity of hue,in particular in mixtures of dyes, and produce improved black prints.

The transfer sheets required as dye donors for the thermotransferprinting process according to the present invention are prepared asfollows. The azo dyes I are incorporated into a suitable organicsolvent, eg. isobutanol, methyl ethyl ketone, methylene chloride,chlorobenzene, toluene, tetrahydrofuran or a mixture thereof, togetherwith one or more binders and possibly further assistants such as releaseagents or crystallization inhibitors to form a printing ink in which thedyes are preferably present in a molecularly dispersed, ie. dissolved,form. The printing ink is then applied to an inert substrate and dried.

Suitable binders for the use of the azo dyes I according to the presentinvention are all materials which are soluble in organic solvents andwhich are known to be suitable for thermotransfer printing, eg.cellulose derivatives such as methylcellulose, hydroxypropylcellulose,cellulose acetate or cellulose acetobutyrate, but in particularethylcellulose and ethylhydroxyethylcellulose, starch, alginates, alkydresins and vinyl resins such as polyvinyl alcohol orpolyvinylpyrrolidone and also in particular polyvinyl acetate andpolyvinyl butyrate. It is also possible to use polymers and copolymersof acrylates or derivatives thereof such as polyacrylic acid, polymethylmethacrylate and styrene-acrylate copolymers, polyester resins,polyamide resins, polyurethane resins or natural resins such as gumarabic.

It is frequently advisable to use mixtures of these binders, for examplemixtures of ethylcellulose and polyvinyl butyrate in a weight ratio of2:1.

The weight ratio of binder to dye is in general from 8:1 to 1:1,preferably from 5:1 to 2:1.

Suitable assistants are for example release agents based onperfluorinated alkylsulfonamidoalkyl esters or silicones, as describedin EP-A-227 092 and EP-A-192 435, and in particular organic additiveswhich stop the transfer dyes from crystallizing out in the course ofstorage or heating of the inked ribbon, for example cholesterol orvanillin.

Inert substrate materials are for example tissue, blotting or parchmentpaper and films made of heat resistant plastics such as polyesters,polyamides or polyimides, which films may also be metal coated.

The inert substrate may additionally be coated on the side facing thethermal printing head with a lubricant in order that adhesion of thethermal printing head to the substrate material may be prevented.Suitable lubricants are for example silicones or polyurethanes asdescribed in EP-A-216 483.

The thickness of the dye transfer is in general from 3 to 30 μm,preferably from 5 to 10 μm.

The receiving medium to be printed, eg. paper, must in turn be coatedwith a binder which receives the dye during the printing process. It ispreferable to use for this purpose polymeric materials whose glasstransition temperatures T_(g) are within the range from 50° to 100° C.,eg. polycarbonates and polyesters. Details may be found in EP-A-227 094,EP-A-133 012, EP-A-133 011, JP-A-199 997/1986 or JP-A-283 595/1986.

EXAMPLES

First, transfer sheets (donors) were prepared in a conventional mannerfrom a polyester sheet 8 μm in thickness coated with an approximately 5μm thick transfer layer of binder B which in each case contained 0.25 gof azo dye I. The weight ratio of binder to dye was in each case 4:1.

The receiving medium to be printed (receiver) was paper about 120 μm inthickness which had been coated with a layer of plastic 8 μm inthickness (Hitachi Color Video Print Paper).

Donor and receiver were placed on top of one another with the coatedfronts next to each other, then wrapped in aluminum foil and heatedbetween two hot-plates at 70°-80° C. for 2 minutes. This operation wasrepeated three times with similar samples at a temperature within therange from 80° to 120° C., the temperature being increased each time.

The amount of dye diffusing into the plastics layer of the receiver inthe course of transfer is proportional to the optical density determinedphotometrically as absorbance A after each heating phase at theabovementioned temperatures.

A plot of the logarithm of the measured absorbances A against thecorresponding reciprocal of the absolute temperature is a straight linefrom whose slope it is possible to calculate the activation energyΔE.sub.τ for the transfer experiment: ##EQU1##

It is additionally possible to infer from the plot the temperature T* atwhich the absorbance attains the value 1, ie. at which the transmittedlight intensity is one tenth of the incident light intensity. The lowerthe temperature T*, the better the thermal transferability of theparticular dye.

The tables which follow divide the azo dyes I used into groups; thevariable P indicates the position of the oxadiazole radical on thephenyl ring of the diazo component in relation to the azo group (o:ortho, m: meta, p: para).

In addition, they list the particular binder B used employing thefollowing abbreviations: EC=ethyl-cellulose, PVB=polyvinyl butyrate,MIX=EC:PVB=2:1.

Further characteristic data are the absorption maxima λ_(max) [nm]measured in methylene chloride and the abovementioned parameters T8[°C.] and ΔE.sub.τ [kcal/mol].

                                      TABLE 1                                     __________________________________________________________________________     ##STR5##                                                                     Ex-                                               λ.sub.max                                                                     ΔE.sub..tau                                                             .                    ample                                                                             P R.sup.1    R.sup.2  R.sup.3   X   Z      B  [nm]                                                                             T*[°C.]                                                                    [kcal/mol]           __________________________________________________________________________    1   p (CH.sub.2).sub.10CH.sub.3                                                                H        CH.sub.3  H   CN     EC 443                                                                              115 17                                                                  MIX   114 21                   2   p (CH.sub.2).sub.14CH.sub.3                                                                H        CH.sub.3  H   CN     EC 444                                                                              110 19                                                                  MIX   107 22                   3   m CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                       H        CH(CH.sub.3)CH.sub.3                                                                    H   COCH.sub.3                                                                           EC 424                                                                              102 15                   4   p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                       H        CH.sub.3  o-NO.sub. 2                                                                       COCH.sub.3                                                                           MIX                                                                              441                                                                              108 15                                                                  EC    127 12                   5   p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                       H        CH.sub.3  H   COCH.sub.3                                                                           MIX                                                                              434                                                                              101 18                   6   p CH.sub.3   (CH.sub.2).sub.5CH.sub.3                                                               NH.sub.2  H   CN     EC*                                                                              429                                                                              125 16                                                                  MIX   110 15                   7   p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                       C.sub.4 H.sub.9                                                                        CH.sub.3  o-NH.sub. 2                                                                       H      EC 437                                                                              112 13                                                                  MIX    99 15                   __________________________________________________________________________     *Weight ratio of binder to dye = 5:1?                                    

                                      TABLE 2                                     __________________________________________________________________________     ##STR6##                                                                     Ex-                                                                           am-                                                λmax                                                                      T* ΔEτ        ple                                                                              P R.sup.1       R.sup.2               X   B  B:F                                                                              [nm]                                                                             [°C.]                                                                     [kcal/mol]           __________________________________________________________________________     8 m (CH.sub.2).sub.2CO.sub.2C.sub.2 H.sub.5                                                     CH.sub.3              H    EC   430                                                                              111                                                                              14                    9 m (CH.sub.2).sub.6CH.sub.3                                                                    CH.sub.3              H   EC    430                                                                               88                                                                               9                   10 p (CH.sub.2).sub.2COOCH.sub.3                                                                 C.sub.2 H.sub.5       H   EC    441                                                                              125                                                                              21                   11 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.2 H.sub.5       H   EC    442                                                                               98                                                                              11                   12 m (CH.sub.2).sub.2COOC.sub.2 H.sub.5                                                          C.sub.2 H.sub.5       H   EC    430                                                                              102                                                                              11                   13 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.2 H.sub.5       o-NO.sub.2                                                                        EC    446                                                                              106                                                                              14                   14 p (CH.sub.2).sub.14CH.sub.3                                                                   (CH.sub.2).sub.2OH    H   MIX   443                                                                              110                                                                              18                   15 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.3 H.sub.7       H   EC    432                                                                              126                                                                              13                                                                MIX      113                                                                              13                   16 m CH.sub.2OPh   CH(CH.sub.3)CH.sub.3  H   EC    428                                                                              127                                                                              11                   17 p (CH.sub.2).sub.14CH.sub.3                                                                   C.sub.4 H.sub.9       H   EC    441                                                                               93                                                                              19                   18 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.4 H.sub.9       H   EC    441                                                                              117                                                                              22                   19 p (CH.sub.2).sub.6 CH.sub.3                                                                   C.sub.4 H.sub.9       H   EC    441                                                                               98                                                                              17                   20 m CH.sub.2OPh   C.sub.4 H.sub.9       H   EC    430                                                                              139                                                                               9                   21 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.4 H.sub.9       H   EC    442                                                                              104                                                                              14                   22 p CH(C.sub.2 H.sub.5)C.sub.4 H.sub.9                                                          C.sub.4 H.sub.9       o-Br                                                                              EC    421                                                                              106                                                                              16                   23 p (CH.sub.2).sub.14CH.sub.3                                                                   (CH.sub.2).sub.4OH    H   MIX   442                                                                               99                                                                              13                   24 m (CH.sub.2).sub.2COOC.sub.2 H.sub.5                                                          (CH.sub.2).sub.3OCH.sub.3                                                                           H   EC    430                                                                               99                                                                              10                   25 m (CH.sub.2).sub.2COOC.sub.2 H.sub.5                                                          (CH.sub.2).sub.2OCH.sub.3                                                                           H   EC    430                                                                              104                                                                              13                                                                MIX       84                                                                              16                   26 m C.sub.2 H.sub.5                                                                             (CH.sub.2).sub.3OCH.sub.2Ph                                                                         H   EC 4:1                                                                              430                                                                               98                                                                              16                   27 m  CH.sub.3     (CH.sub.2).sub.3OCH.sub.2Ph                                                                         H   EC 4:1                                                                              430                                                                               99                                                                               9                   28 p CH.sub.3      (CH.sub.2).sub.3O(CH.sub.2).sub.4OCOCH.sub.3                                                        H   EC 4:1                                                                              440                                                                              100                                                                              11                                                                MIX                                                                              4:1    88                                                                              17                   29 m CH.sub.3      (CH.sub.2).sub.3O(CH.sub.2).sub.4OCOCH.sub.3                                                        H   EC 4:1                                                                              430                                                                              100                                                                               9                                                                MIX                                                                              4:1    79                                                                              19                   30 m CH.sub.3      (CH.sub.2).sub.3OC(CH.sub.3).sub.2(CH.sub.2).sub.2OCH.s                       ub.3                  H   EC 4:1                                                                              430                                                                              100                                                                              20                   31 p CH.sub.3      (CH.sub.2).sub.5CH.sub.3                                                                            H   EC 4:1                                                                              441                                                                              112                                                                              13                   __________________________________________________________________________

We claim:
 1. A process for transferring an azo dye by diffusion from atransfer to a plastic-coated receiving medium with the aid of a thermalprinting head, which comprises using for this purpose a transfer onwhich there is or are situated one or more azo dyes of the formula I##STR7## in which the substituents have the following meanings: R¹ andR² are eachhydrogen; alkyl, alkoxyalkyl, alkanoyloxyalkyl,alkoxycarbonyloxyalkyl, alkoxycarbonylalkyl, haloalkyl, hydroxyalkyl orcyanoalkyl, which may each contain up to 20 carbon atoms and besubstituted by phenyl, C₁ -C₄ alkylbenzyloxy, C₁ -C₄ -alkoxybenzyloxy,C₁ -C₄ alkylphenyl, C₁ -C₄ alkoxyphenyl, halophenyl, benzyloxy, C₁ -C₄alkylbenzyloxy, C₁ -C₄ -alkoxybenzyloxy, halobenzyloxy, halogen,hydroxyl or cyano; phenyl or cyclohexyl, which may each be substitutedby C₁ -C₁₅ -alkyl, C₁ -C₁₅ -alkoxy, halogen or benzyloxy; or a radicalof the general formula II:

    [--W--O].sub.n --R.sup.4                                   II

whereW is identical or different C₂ -C₆ -alkylene, n is from 1 to 6 andR⁴ is C₁ -C₄ -alkyl or phenyl which may each be substituted by C₁ -C₄-alkyl or C₁ -C₄ -alkoxy; R³ is hydrogen, amino, hydroxyl or alkyl; X ishydrogen chlorine, bromine, nitro, methyl, phenoxy, tolyloxy,dimethylphenyloxy, chlorophenoxy or C₁ -C₄ -alkoxy; Y is hydrogen,chlorine or bromine; and is hydrogen, acetyl, carbamoyl or cyano.
 2. Aprocess as claimed in claim 1, wherein the azo dye used has the formulaIa ##STR8##
 3. A process as claimed in claim 1, wherein the azo dye usedhas the formula Ib ##STR9## in which the variables have the followingmeanings: R^(1') and R^(2') are each:hydrogen; alkyl, alkoxyalkyl,alkanoyloxy or alkoxycarbonylalkyl, which may each contain up to 15carbon atoms and be substituted by phenyl, C₁ -C₄ -alkylphenyl, C₁ -C₄-alkoxyphenyl, hydroxyl or cyano; or a radical of the general formulaIII

    [--(CH.sub.2).sub.3 --O].sub.p --(CH.sub.2).sub.2 --O].sub.q --R.sup.4' III

where p is 0 or 1, q is from 1 to 4, and R^(4') is C₁ -C₄ -alkyl orphenyl.
 4. The process of claim 1, wherein said the plastic of saidplastic-coated receiving medium is selected from the group consisting ofpolycarbonates and polyesters or a mixture thereof.